Japan also agreed to give the Thais some $15 million to help increase food production, and is looking for ways to ease the Thais' burden of Indochinese refugees.

* A group of Vietnamese lawmakers came to Tokyo recently to ask for a resumption of Japanese economic aid, suspended when Hanoi invaded Cambodia. Prime Minister Zenko Suzuki gave a flat no. This was seen as a firm sign of a strong stand against Vietnam and in favor of noncommunist Southeast Asia.

The Japanese have had to work hard for acceptance in Southeast Asia. Their neighbors remember World War II, and fear and suspicion linger.

More recently Japan has seemed to threaten economic domination and even exploitation. (Seven years ago, a Southeast Asian tour by then Prime Minister Kakuei Tanaka triggered anti-Japanese riots.)

Prime Minister Suzuki made a sweep through all five members of ASEAN, the Association of Southeast Asian Nations, early this year. He scored a number of points, and Tokyo has been working hard since to translate his promises into action.

Although Japan's overtures have extended to all ASEAN members - Malaysia, Indonesia, the Philippines, Singapore, and Thailand - the Thais are getting special attention to help counter the effects of Vietnamese troops in next-door Cambodia.

Under its regular aid program, Japan has already committed almost $240 million to Thailand for the current fiscal year ending next March 31.

On his recent visit to Tokyo, Thai Premier Prem Tinsulanonda also won a promise of a $65 million low-interest government loan for an offshore butane and propane plant in the Gulf of Siam. (The possibility of export to Japan obviously has not been overlooked.)

Government sources say it is likely another $130 million will be contributed to the project from the Export-Import Bank.

Another big concession was Japan's promise to try to correct a chronic trade surplus with Thailand.

Tokyo will try to stimulate import of Thai goods and encourage Japanese investment in production facilities in Thailand.

Japan's refugee assistance program is also under review, in fact.

By contrast, Tokyo's relations with Vietnam remain frosty. Japan has lobbied hard for support for an ASEAN-proposed conference to discuss a political solution of the Cambodian problem. Vietnam, it has made been made clear, won't get one yen from Japan until it agrees to attend such a conference.

Some suggest that Tokyo, in fact, could do a lot more diplomatically to achieve a solution of the Cambodian issue. After all, it is pointed out, it is a nonmilitary neutral in Indochina, with diplomatic ties with both China and Vietnam. Thus Japan can play a unique ''bridging'' role.